Strip reduction measurement device and control system using the same for rolling mill

ABSTRACT

Two measuring coils are disposed on the oncoming and outrunning sides of a rolling mill stand respectively and an electrically conductive workpiece passes through the coils and stand. One oscillator including each coil responds to a pass of the workpiece through the associated coil to change in oscillation frequency. A difference between changes in frequency of both oscillators or a ratio between them is measured to provide a measure of a difference or a ratio between cross sectional areas of the workpiece in the planes of both coils.

United States Patent [1 1 Yoshitani et al.

1451 Jan. 28, 1975 1 STRIP REDUCTION MEASUREMENT DEVICE AND CONTROL SYSTEM USING THE SAME FOR ROLLING MILL [75] Inventors: Yutaka Yoshitani, Tokyo; Yoshiharu I-lamazaki, Amagasuki, both of Japan [22] Filed: Dec. 27, 1972 [21] Appl. No.1 318,770

2,275,509 3/1942 Dahlstrom 72/9 3.142.796 7/1964 Goldberg et a1. 324/34 R 3.444.713 5/1969 Barnikel 72/8 3.526.113 9/1970 McNaugher 72/8 FOREIGN PATENTS OR APPLICATIONS 1.505.716 12/1967 France 324/34 R 758,541 10/1956 Great Britain 324/34 T Primary ExuminerMilton S. Mehr Attorney, Agent, or FirmRobert E. Burns [57] ABSTRACT Two measuring coils are disposed on the oncoming [30] F i A i i priority r and outrunning sides of a rolling mill stand respec- Dec 30 971 h an 47 2372 tlvely and an electrically conductive workpiece passes p through the coils and stand. One oscillator including [52] U S CI 72/ 72/16 each coil responds to a pass of the workpiece through [51] I 37/00 the associated coil to change in oscillation frequency. [58] fie'ld H l2 16, A difference between changes in frequency of both oscillators or a ratio between them is measured to provide a measure ofa difference or a ratio between cross [56] References Cited sectional areas of the workpiece in the planes of both c 'l UNITED STATES PATENTS 5 1,969,536 8/1934 Winne 1. 72/240 x 8 Clams, 5 Drawing Figures Ji 4 10 HIGH FREQU E NCY OSCILLATOR PATENTEI] JAN28 I975 SHEET 10F 2 HIGH FREQUENCY SOURCE BALANCE DETECTOR FIG.

W, 22 HIGH EQUENCY OSCILLATOR OP fl 24 f +Af L HETE YNE l CY CON ION FR OSCILLATOR TRA OE 1N1 'Afg FREQUENCY- 126 TO VOLTAGE CONVERTER VERS NSDU HI FREQ I/( HIGH QUENCY LLATOR ERODYNE VERSI ON SER HET CON TRANSDU REFERE FRE GEN NCE QUENCY ERATOR Afg DIVIDER IAfz/ n HIGH FRE ENCY OS ATOR HETERODYNE CONVERSION TRANSDUSER STRIP REDUCTION MEASUREMENT DEVICE AND CONTROL SYSTEM USING THE SAME FOR ROLLING MILL CROSS REFERENCE TO RELATED APPLICATIONS Reference is made to the following applications:

Ser..No. 32,551 entitled System For Measuring Sectional Area, filed by H. Kishimoto, K. Kobayashi, Y. I-Iamazaki and M. Danno on Apr. 28, 1970 and abondoned.

Ser. No. 25,668 entitled Flow Meter and filed by H, Kishimoto, K. Kobayashi, Y. Hamazaki and M. Danno on Apr. 6, 1970, now U.S. Pat. No. 3,715,919.

BACKGROUND OF THE INVENTION This invention relates to metal rolling mills and more particularly to a measurement device for measuring a magnitude of strip reduction effected by such a mill and a mass flow control system using the same for a strip rolling mill.

In conventional strip rolling rolls, the thickness meter could be utilized to permit a sheet metal passed through each of rolling stands thereof to be adjusted in reduction on the basis of the particular value ofa thickness thereof measured by the meter. However metals such as hoop steels are complicated in cross-sectional profile so that the cross-sectional area and profile thereof has been previously difficult to be determined. In order to overcome this difficulty, it has been already proposed to utilizean electromagnetic coil through which an bar-shaped metal is passed as disclosed in U.S. application Ser. No. 32,551 above cited. According to the cited application, an electrically conductive body to be measured is adapted to pass through an electromagnetic coil tochange the inductance thereof. This change in inductance of the coil provides a measure of the cross-sectional area of the conductive body. The measure has encountered various problems. For example, a variation in cross-sectional area of the conductive body has caused a very small change in inductance of the measuring coil and also the measurement has been affected by a change in cross-sectional profile of the conductive body, the disposition of the body eccentric to the center of the coil etc. Further if it has been attempted to measure directly the cross-sectional area of the conductive body, then it has been required to highly increase the stability of an high frequency oscillator involved. In addition, it has been necessary to consider the cross-sectional area of the conductive body and the like.

SUMMARY OF THE INVENTION Accordingly it is a general object of the present invention to solve the problems as above described.

It is a principal object of the present invention to provide an improved device for measuring a magnitude of reduction of area of an electrically conductive workpiece to be rolled effected by a rolling mill with a high accuracy of measurement.

It is another object of the invention to provide a new and improved device for accurately measuring a change in cross-sectional area of an electrically conductive workpiece due to the rolling thereof effected by a metal rolling mill, that is to say, a difference in or a ratio of cross-sectional area of the workpiece between before and after rolling.

It is still another object of the invention to provide an improved device for accurately measuring a strip reduction, that is, a difference in or a ratio of reduction of area between a hoop steel having a complicated cross-sectional profile before rolled and the same after rolled by a rolling mill.

It is a further object of the present invention to provide an improved measurement device operative in the basis of a change in inductance of an electromagnetic coil involved to measure a reduction of a workpiece to be rolled effected by a rolling mill with substantially free from the effects of a change in cross-sectional profile of the workpiece, the disposition thereof eccentric to the center of the coil etc.

It is another object to provide an improved device for measuring a reduction of a workpiece effected by a metal rolling mill with a small error in measurement caused from the instability of an oscillation frequency produced by a high frequency oscillator used to measure a change in inductance of the associated electromagnetic coil.

It is an additional object of the present invention to provide a new and improved control system for controlling a metal rolling mill for rolling a workpiece complicated in cross-sectional profile, with a high accuracy and on the basis of a value of reduction of area of the workpieced provided by the reduction measuring device as described ineach of the preceding paragraphs.

The present invention accomplishes the above cited objects by the provision of a measurement device for measuring a reduction of area ofa workpiece, comprising a rolling stand, an electrically conductive workpiece rolled by the rolling stand, measuring coil means responsive to the pass of the electrically conductive workpiece ther'ethrough to change in inductance thereof, and means for detecting a, change in inductance of the measuring coil means, characterized in that the measuring coil means includes a pair of electromagnetic coils disposed on the oncoming and outrunning sides of the rolling stand in coaxial relationship in the direction of pass of the workpiece, and there is provided means for producing a measure of a reduction of area of the workpiece from the changes in inductances of the two electromagnetic coils.

The means for producing a measure of a reduction of area may preferably comprise one high frequency oscillator including each electromagnetic coil and changing in oscillation frequency in responsive to the pass of the workpiece through the associated coil, and means for producing a difference or a ratio between the changes in the oscillation frequencies to provide a measure of a difference or a ratio between cross-sectional areas of the workpiece in the plance of the electromagnetic coils.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagrammatic view useful in explaining the fundamental principles of a device for measuring a cross-sectional area of an electrically conductive body for use with the present invention;

FIG. 2 is a schematic view of a device for measuring a difference between an entry and a delivery thickness of a workpiece rolled by a rolling mill stand in accordance with the principles of the present invention using a heterodyne method;

FIG. 3 is a schematic view of a device for measuring a ratio between an entry and a delivery thickness of a workpiece rolled by a rolling mill stand in accordance with the principles of the present invention using an impedance bridge;

FIG. 4 is a view similar to FIG. 3 but illustrating a modification of the invention using a heterodyne method; and

FIG. 5 is a schematic view of a control system for controlling a strip rolling mill by using the arrangement shown in FIG. 4.

Throughout the Figures like reference numerals designated the corresponding or similar components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the present invention, the fundamental principles of a measurement device used therewith to measure a cross-sectional area of a body of an electrically conductive material will now be described in conjunction with FIG. 1. The arrangement illustrated comprises an electromagnetic coil 10, a body of electrically conductive material 12 passed through the coil 10 without physically contacting the coil 10. A high frequency oscillator 14 cooperates with the coil 10 to establish a high frequency magnetic field having a frequency of faround the coil 10. It is now assumed that the electrically conductive body 12 has no magnetic property only for purposes of illustration and has an electric conductively of If the body 10 is of a magnetic material, it can be put at a temperature above its Curie temperature to be nonmagnetic. Due to its skin effect, the high frequency magnetic field can penetrate into the conductive body 12 to a depth of 6 (see FIG. 1) expressed by the following equation where w angular frequency of the high frequency magnetic field equal to its frequency fmultiplied by 211' p. permeability of the material of the body 12, under the assumed condition, equal to 41r'l0"henry per meter.

From the above equation it is apparent that the penetrated or skin depth 8 can be negligibly small by sufficiently increasing the frequency f That is, the high frequency magnetic field can be substantially completely excluded from the conductive body 10 by means of the skin effect by sufficiently increasing its frequency f Under these circumstances,.the coil 10 changes in its equivalent inductance. Assuming that the skin depth is negligibly small, this change in inductance holds the relationship where L0 inductance of the coil having extending therethrough no conductive body.

L change in inductance of the coil 10 due to the conductive body extending therethrough.

S0 cross-sectional area of the coil 10 S cross-sectional area of the conductive body 12 A constant determined by the shape of the coil 10 a another constant determined by the shape of the coil 10.

Therefore it will be appreciated that a change in inductance of the coil 10 can be measured thereby to determine the cross-sectional area of the conductive body 12 in the plane of the coil 10.

While the conductive body 12 extending through the coil 10 has been described to be of a nonmagnetic material, it may be of a magnetic material as far as its permeability remains substantially unchanged as compared with a reference body used for purpose of calibration.

Further information may be found in U.S. application Ser. No. 32,551 as above cited. Also flow meters utilizing the arrangement of FIG. 1 are described and claimed in U.S. application Ser. No. 25,668 as also cited above.

The arrangement as shown in FIG. 1 can effectively determine a cross-sectional area of any electrically conductive body even though complicated in its crosssectional profile. However, the utilization of such an arrangement to determine the cross-sectional area of electrically conductive bodies has given rise to various problems. For example, a variation in cross-sectional area of the conductive body has only caused an extremely small change in impedance of the coil. Also a change in impedance of the coil has been able to be affected by a variation in cross-sectional profile of the conductive body, the disposition of the conductive body eccentric to the center of the coil etc. Further in order to effect the direct measurement of crosssectional areas, it is required to much increase the stability of a high frequency oscillator involved and also it is necessary to consider the cross-sectional profile of electrically conductive bodies to be measured.

The present invention contemplates to solve those problems in simple effective manner. According to the principles of the present invention, a pair of measuring electromagnetic coils are disposed on the oncoming and outrunning sides of a rolling stand of a strip rolling mill respectively and a workpiece to be rolled is arranged to pass through both coils. Then the arrangement of FIG. 1 is utilized to determine a magnitude of reduction of area of the workpiece effected by the rolling stand, that is to say, a difference or a ratio between cross-sectional areas thereof before and after passed through the stand. Thus the present invention does not directly measure an absolute value of the crosssectional area of the workpiece.

FIG. 2 shows one embodiment of the present invention wherein a difference between a cross-sectional area of a workpiece before it is rolled and that after it has been rolled is measured in accordance with a heterodyne method. The arrangement illustrated comprises a rolling mill stand represented by a pair of work rolls l6 and a pair of measuring electromagnetic coils l0 and 18 disposed in coaxially relationship on the oncoming and outrunning sides of the rolling stand 16. A workpiece 12 to be rolled passes first through the coil 10 on the oncoming side of the stand 16 and then through between the work rolls 16 to be reduced to a desired strip thickness. Thereafter the rolled workpiece 12 passes through the coil 18 on the outrunning side of the stand 16.

The coil 10 forms one part ofa high frequency oscillator 20 for normally generating a predetermined fixed frequency of 13,. Similarly the coil 18 forms one part of another high frequency oscillator 22 for normally generating a predetermined fixed frequency of f The workpiece l2 enters the coil to change the inductance thereof to change the oscillation frequency of the oscillator from its value off to its value off Af,. When the workpiece l2 enters the coil 18, the oscillator 22 is similarly changed from the frequency of f g to a frequency off Af The high frequency of f, Afi from the oscillator 20 is applied to a heterodyne conversion transducer 24 as does the frequency of fog fg from the oscillator 22. The heterodyne conversion transduce 24 includes a pair of oscillators for generating the frequencies of f. and f respectively, heterodyne means for producing a difference frequency between the frequencies off Af, and f that is, a frequency of Af and producing a differenc frequency Af between the frequencies of Af and f g and another heterodyne means for producing a difference frequency of Af Af from the heterodyned frequencies of Afi and Af although those components are not illustrated. The difference frequency of Af, Af is supplied to a frequency-tovoltage converter 26 to be converted to a corresponding voltage.

The relationship between the cross-sectional area of the workpiece 12 in the plane of the coil 10 on the oncoming side of the rolling stand 16 and the oscillation frequency provided from the oscillator 20 can be expressed by the following equation:

Ml M r ft/fol where B is a constant. Similarly, for the coil 18 on the outrunning side of the stand 16, the following relationship is held:

where B is a constant. The difference frequency of Af, Afi provided by the heterodyne conversion transducer 24 is expressed by where K =f /B 1/8,, 12 /192 1/502 It will readily be understood that 12,, f g, S and S can be preselected to fulfil the relationship Therefore voltage provided by the frequency-tovoltage converter 26 is proportional to the difference frequency (Af, Af and provides a measure of a difference in cross sectional area of the workpiece 12 between before and after its rolling. I

The arrangement of FIG. 2 provides a difference between the oscillation frequencies before and after a workpiece is rolled, and therefore the reduction of area of the workpiece can be determined with small errors caused from the abovementioned factors. 7

In the arrangement of FIG. 2, each oscillator has a rate of change in frequency Aflf as small as about from 10 to 10 Thus if it is attempted to measure an absolute value of a cross-sectional area of a workpiece then it is required to highly stabilize measurement devices involved which is very difficult in such surroundings as rolling mills. In such surroundings, an arrangement as shown in FIG. 3 can be effectively used in order to minimize the effects of a variation in a high oscillation frequency supplied to the associated electromagnetic coil and a change in supply voltage upon measurement.

The arrangement illustrated in FIG. 3 comprises a pair of electromagnetic coils 10 and 18 disposed with respect to a rolling mill stand 16 in the same manner as above described in conjunction with FIG. 2, and a rheostat 28 connected across the coils l0 and 18 at one end. The other ends of both coils l0 and 18 are connected to ground and a tap on the rheostat 28 is connected to one end of a balance detector 30 connected at the other end to ground. Thus the coils l0 and 18 and the rheostat 28 form an impedance bridge energized by a source of high frequency 32 connected across the rheostat 28. The frequency of the source 32 can be appropriately determined in accordance with a cross-sectional area of the particular workpiece to be rolled. It has been found that the frequency of the source 32 ranges from 100 Kilohertz to 5 megahertz with satisfactory results.

In the arrangement of FIG. 3 a ratio between a crosssectional area of the workpiece in the plane of the coil 10 and that in the plane of the coil 18 is determined by a ratio between both portions into which the entire resistance of the rheostat 28 is divided by the tap thereon when the impedance bridge is in balanced state.

FIG. 4 shows another modification of the present in- I vention wherein a ratio of cross-sectional area for a workpiece to be rolled is determined by a heterodyne method. A pair of oscillators 20 and 22 are identical to those shown in FIG. 2 except for the generation of a common frequency of j; in the absence of a workpieces within the associated coils l0 and 18. A reference frequency generator 34 supplies a reference frequency equal to that common frequency f, to a pair of heterodyne conversion transducer 24' and 24 having applied thereto the frequencies from the oscillators 20 and 24, respectively.

In the arrangement of FIG. 4, the workpiece 12 passes through the coil 10 to cause the associated oscillator 20 to changethe oscillation frequency by a value of Af, which is, in turn, detected by the heterodyne conversion transducer 24'. Similarly a pass of the rolled workpiece 12 through the coil 18 causes a change in frequency Af of the associated oscillator 22. This change in frequency is detected by the heterodyne conversion transducer 24. Then those changes in frequency detected by the heterodyne conversion transducer 24' and 24 are supplied to a divider 36 to provide a ratio of Af /Afl. Since the relationship Afz/Af k S /S where k is a constant is held, the output from the divider 36 provides a measure of a ratio of crosssectional area of the workpiece on the oncoming side to that on the outrunning side of the rolling stand 16.

Referring now to FIG. 5, there is illustrated a control system for use with a continuous strip rolling mill to control both a rolling speed and a strip reduction on each of rolling mill stands through the utilization of the arrangement as shown in FIG. 4. In FIG. 5 a continuous strip rolling mill is generally designated by the referencenumeral 50 and includes a plurality of rolling stands disposed in tandem only three of which are shown and labelled STAND l, STAND II" and STAND Ill, respectively. Each of the rolling stands includes a pair of work rolls 16, for providing a desired strip reduction as a workpiece l2 successively passes through each of the several stands, a screwdown mechanism 52 for detecting a screwdown position for the work rolls l6 and regulating it and a speed mechanism 54 for detecting a rolling speed of the work rolls 16 and regulating it. Only for purposes of illustration those detectors and regulators are not shown. Further each of the rolling stands are operatively associated with one arrangement as shown in FIG. 4 with a single reference frequency oscillator 34 connected to all the heterodyne conversion transducers 24 and 24". All the screwdown mechanism 52, the speed mechanisms 54 and dividers 36 are connected to a programmed digital process control computer 56.

A workpiece 12 successively and alternately passes through the coils and rolling stands while the process control computer 56 receives the actual data for each of the rolling stands 16 from the associated screwdown mechanism 52, speed mechanism 54 and divider 36 to control the rolling process following the law that a mass flow for the workpiece 12 is maintained constant throughout the rolling mill 50. More specifically, assuming that on each stand, S and S designate the entry and delivery cross-sectional areas of the workpiece and V and V designate the entry and delivery speeds of thereof, the rolling process should be controlled to meet the following relationship SMIVI m z If the divider 36 operatively coupled to a particular rolling stand determines a change in the entry crosssectional area from the value of S to a value of S for any reason, or reasons, the computer 56 commands the associated screwdown mechanism 52 and/or speed mechanism 54 to control the rotational speed or the cross sectional area on the stand in accordance with I where the V, and V are provided from the associated speed mechanisms 54 and known. In this way, the workpiece is stably and continuously rolled by the successive rolling stands.

The control system has been illustrated and described to use the arrangement of FIG. 4 but it is to be understood that the control system may be operatively associated with any desired strip reduction measuring device such as shown in FIGS. 2 and 3.

While the present invention has been illustrated and described in conjunction with a few preferred embodiments thereof it is to be understood that numerous changes and modifications may be resorted to without departing from the spirit and scope of the invention.

We claim:

1. A device for measuring the reduction of the crosssectional area of a workpiece effected by a rolling mill, comprising: a rolling stand, a pair of electromagnetic coils disposed on the oncoming and outgoing sides of the rolling stand respectively and receptive therethrough of an electrically conductive workpiece rolled by the rolling stand during use and passing through the electromagnetic coils to change the inductances thereof as a function of the cross sectional area, detector means for detecting the deviation in inductance of each of the electromagnetic coils due to the passing of the workpiece therethrough and means for developing a signal determinative of the reduction of the crosssectional area of the workpiece effected by the rolling stand solely as a function of the inductance deviations of the electromagnetic coils.

2. A strip reduction measuring device as claimed in claim 1, wherein the means for developing said signal determinative of the reduction of area includes means for developing said signal proportional to the difference between the two inductance deviations which is a function of the difference between cross-sectional areas of the workpiece in the planes of theelectromagnetic coils.

3. A strip reduction measuring device as claimed in claim 1, wherein the means for developing said signal determinative of the reduction of area includes means for developing said signal proportional to the ratio of the two inductance deviations which is a function of the ratio of cross-sectional areas of the workpiece in the plane of each of the electromagnetic coils.

4. A device for measuring the reduction of the crosssectional area ofa workpiece effected by a rolling mill, comprising: a rolling stand, a pair of electromagnetic coils disposed on the oncoming and outrunning sides of the rolling stand respectively and receptive therethrough of an electrically conductive workpiece rolled by the rolling stand during use and passing through the electromagnetic coils to change the inductances thereof, a first oscillator circuit including one of the electromagnetic coils and responsive to the change in inductance thereof to effect a deviation in the oscillation frequency of the circuit, a second oscillator circuit including the other electromagnetic coil and responsive to the change in inductance thereof to effect a deviation in the oscillation frequency of the second oscillation circuit, and means for developing a signal determinative of the reduction of the cross-sectional area of the workpiece effected by the rolling stand solely as a function of the frequency deviations of the two oscillator circuits.

5. A strip reduction measuring device as claimed in claim 4, wherein the means for developing said signal determinative of the reduction of area comprises a heterodyne conversion transducer including a pair of reference oscillators normally equal in oscillation frequency to that of the first and second oscillator circuits respectively, and heterodyne means for first producing two signals corresponding to the deviation in frequency between the first and second oscillators and their associated reference oscillators respectively and then producing a signal corresponding to the frequency difference between the deviation frequencies, and a frequency-to-voltage converter for converting the frequency difference to said signal determinative of the reduction of area having a corresponding voltage providing a measure of a difference between cross-sectional areas of the workpiece in the planes of the electromagnetic coils.

6. A strip reduction measuring device as claimed in claim 4, wherein the first and second oscillator circuits are equal in oscillation frequency to each other and wherein the means for developing said signal determinative of the reduction of the cross-sectional area comprises a reference frequency generator for generating a signal having a frequency equal to that of the first and second oscillator circuits, a first heterodyne conversion transducer for developing a first signal representative of the deviation in oscillation frequency between the second oscillator circuit and the reference frequency generator, and divider means for developing a signal proportional to the ratio between the of the first and second signals corresponding to the ratio of the frequency deviations developed by the first and second heterodyne conversion transducers thereby providing a measure of the ratio between cross-sectional areas of the workpiece in the plane of each of the electromagnetic coils.

7. A control system for a rolling mill comprising: a rolling stand; a pair of electromagnetic coils, one disposed on the oncoming side and one disposed on the outgoing side of the rolling stand respectively and each receptive of an electrically conductive workpiece rolled by the rolling stand during use and passing through the electromagnetic coils to change inductance of each; detector means for detecting the deviation in inductance of each of the electromagnetic coils effected-by the passing of the workpiece therethrough; means for developing a signal determinative of the reduction of the cross-sectional area of the workpiece effected by the rolling stand at each position of the coil solely as a function of the inductance deviations of both coils; means for detecting the rolling speed of the workpiece on the oncoming side of the rolling stand and for developing a speed signal proportional thereto, means for controlling a rolling speed on the rolling stand; and means for controlling the rolling speed controlling means in response to both said signal determinative of the reduction of area and said speed signal from speed detecting means to maintain a constant mass flow for the workpiece on the oncoming and outgoing sides of the rolling stand.

8. A control system for a rolling mill comprising: at least one rolling stand; at least two electromagnetic coils including a first coil and a second coil receptive of an electrically conductive workpiece rolled by the rolling stand during use and passing through the first and second coils disposed on the oncoming and outgoing sides of the rolling stand respectively to change inductances thereof as a function of the cross sectional area of the workpiece, first oscillator means including the first coil and responsive to the change in inductance thereof to effect a deviation in its oscillation frequency; means for developing a signal having a reference frequency; first means for developing a first signal proportional to the frequency difference between the oscillation frequencies of the first oscillator means and the reference frequency means; second means for developing a second signal proportional to the frequency difference between the oscillation frequencies of the reference and second oscillator means; divider means receptive of the first and second signals for developing a third signal corresponding to the ratio thereof solely proportional to the frequency deviations of the first and second oscillator means; third means for detecting the speed of the workpiece on the oncoming side of the rolling stand and for developing a speed signal proportional thereto; means for controlling the rolling speed on the rolling stand; and means for controlling the rolling speed controlling means in response to both said third signal from said divider means and said speed signal from the third means to maintain a constant mass flow for the workpiece on the oncoming and outgoing sides of the rolling stand. 

1. A device for measuring the reduction of the cross-sectional area of a workpiece effected by a rolling mill, comprising: a rolling stand, a pair of electromagnetic coils disposed on the oncoming and outgoing sides of the rolling stand respectively and receptive therethrough of an electrically conductive workpiece rolled by the rolling stand during use and passing through the electromagnetic coils to change the inductances thereof as a function of the cross sectional area, detector means for detecting the deviation in inductance of each of the electromagnetic coils due to the passing of the workpiece therethrough and means for developing a signal determinative of the reduction of the cross-sectional area of the workpiece effected by the rolling stand solely as a function of the inductance deviations of the electromagnetic coils.
 2. A strip reduction measuring device as claimed in claim 1, wherein the means for developing said signal determinative of the reduction of area includes means for developing said signal proportional to the difference between the two inductance deviations which is a function of the difference between cross-sectional areas of the workpiece in the planes of the electromagnetic coils.
 3. A strip reduction measuring device as claimed in claim 1, wherein the means for developing said signal determinative of the reduction of area includes means for developing said signal proportional to the ratio of the two inductance deviations which is a function of the ratio of cross-sectional areas of the workpiece in the plane of each of the electromagnetic coils.
 4. A device for measuring the reduction of the cross-sectional area of a workpiece effected by a rolling mill, comprising: a rolling stand, a pair of electromagnetic coils disposed on the oncoming and outrunning sides of the rolling stand respectively and receptive therethrough of an electrically conductive workpiece rolled by the rolling stand during use and passing through the electromagnetic coils to change the inductances thereof, a first oscillator circuit including one of the electromagnetic coils and responsive to the change in inductance thereof to effect a deviation in the oscillation frequency of the circuit, a second oscillator circuit including the other electromagnetic coil and responsive to the change in inductance thereof to effect a deviation in the oscillation frequency of the second oscillation circuit, and means for developing a signal determinative of the reduction of the cross-sectional area of the workpiece effected by the rolling stand solely as a function of the frequency deviations of the two oscillator circuits.
 5. A strip reduction measuring device as claimed in claim 4, wherein the means for developing said signal determinative of the reduction of area comprises a heterodyne conversion transducer including a pair of reference oscillators normally equal in oscillation frequency to that of the first and second oscillator circuits respectively, and heterodyne means for first producing two signals corresponding to the deviation in frequency between the first and second oscillators and their associated reference oscillators respectively and then producing a signal corresponding to the frequency difference between the deviation frequencies, and a frequency-to-voltage converter for converting the frequency difference to said signal determinative of the reduction of area having a corresponding voltage providing a measure of a difference between cross-sectional areas of the workpiece in the plAnes of the electromagnetic coils.
 6. A strip reduction measuring device as claimed in claim 4, wherein the first and second oscillator circuits are equal in oscillation frequency to each other and wherein the means for developing said signal determinative of the reduction of the cross-sectional area comprises a reference frequency generator for generating a signal having a frequency equal to that of the first and second oscillator circuits, a first heterodyne conversion transducer for developing a first signal representative of the deviation in oscillation frequency between the second oscillator circuit and the reference frequency generator, and divider means for developing a signal proportional to the ratio between the of the first and second signals corresponding to the ratio of the frequency deviations developed by the first and second heterodyne conversion transducers thereby providing a measure of the ratio between cross-sectional areas of the workpiece in the plane of each of the electromagnetic coils.
 7. A control system for a rolling mill comprising: a rolling stand; a pair of electromagnetic coils, one disposed on the oncoming side and one disposed on the outgoing side of the rolling stand respectively and each receptive of an electrically conductive workpiece rolled by the rolling stand during use and passing through the electromagnetic coils to change inductance of each; detector means for detecting the deviation in inductance of each of the electromagnetic coils effected-by the passing of the workpiece therethrough; means for developing a signal determinative of the reduction of the cross-sectional area of the workpiece effected by the rolling stand at each position of the coil solely as a function of the inductance deviations of both coils; means for detecting the rolling speed of the workpiece on the oncoming side of the rolling stand and for developing a speed signal proportional thereto, means for controlling a rolling speed on the rolling stand; and means for controlling the rolling speed controlling means in response to both said signal determinative of the reduction of area and said speed signal from speed detecting means to maintain a constant mass flow for the workpiece on the oncoming and outgoing sides of the rolling stand.
 8. A control system for a rolling mill comprising: at least one rolling stand; at least two electromagnetic coils including a first coil and a second coil receptive of an electrically conductive workpiece rolled by the rolling stand during use and passing through the first and second coils disposed on the oncoming and outgoing sides of the rolling stand respectively to change inductances thereof as a function of the cross sectional area of the workpiece, first oscillator means including the first coil and responsive to the change in inductance thereof to effect a deviation in its oscillation frequency; means for developing a signal having a reference frequency; first means for developing a first signal proportional to the frequency difference between the oscillation frequencies of the first oscillator means and the reference frequency means; second means for developing a second signal proportional to the frequency difference between the oscillation frequencies of the reference and second oscillator means; divider means receptive of the first and second signals for developing a third signal corresponding to the ratio thereof solely proportional to the frequency deviations of the first and second oscillator means; third means for detecting the speed of the workpiece on the oncoming side of the rolling stand and for developing a speed signal proportional thereto; means for controlling the rolling speed on the rolling stand; and means for controlling the rolling speed controlling means in response to both said third signal from said divider means and said speed signal from the third means to maintain a constant mass flow for the workpiece on the oncoming and outgoing sides of the rolling stand. 